Report on St. Helens (United States) — September 1980
Scientific Event Alert Network Bulletin, vol. 5, no. 9 (September 1980)
Managing Editor: David Squires.
St. Helens (United States) Harmonic tremor and vapor emission; 18 May material remains in upper atmosphere
Please cite this report as:
Global Volcanism Program, 1980. Report on St. Helens (United States) (Squires, D., ed.). Scientific Event Alert Network Bulletin, 5:9. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198009-321050
46.2°N, 122.18°W; summit elev. 2549 m
All times are local (unless otherwise noted)
September activity was limited to slow outward movement of the N flank, minor seismicity, and weak vapor emission. No eruption was associated with the brief earthquake swarm of 4 September, and seismicity was limited to a few very minor events through mid-month. Outward movement of the rampart N of the inner crater continued, averaging 1.2 cm/day 26 August-9 September (in contrast to the 1.5 m/day movement of the N-flank "bulge" prior to 18 May). Several small jets of ash were ejected from a vent near the base of the lava dome on 9 September. Each burst lasted about 30 minutes, but consisted of only a few cubic meters of ash. Weather and instrument problems limited data on CO2 and SO2 emission rates. Early September CO2/SO2 ratios ranged from 4.7 on the 5th to 2.4 on the 9th, with a maximum CO2 emission rate of 5,000 t/d, on the 6th.
Many large avalanches occurred on the crater walls during the afternoon and early evening of 12 September, but no accompanying seismic activity was recorded. Avalanching declined during the night of 12-13 September. Ground deformation measurements indicated that outward movement of the crater's N rampart had slowed to less than 1 cm/day by mid-September. Several radial fissures in the lava dome within the inner crater widened slightly in the 2 weeks since they first developed about 9 September. The USGS believes that this widening was probably related to the N rampart movement. No dome growth has been detected since 11 August, but some incandescent areas were visible on the dome on 23 September. No significant seismic or eruptive activity was recorded in mid-September.
On 24 September at 0917 a gray gas plume rose from the volcano to about 3 km altitude, just clearing the crater rim, then drifted to the S. Vigorous gas emission lasted about an hour, but there was little or no ash in the plume. No seismicity accompanied this event, but low-amplitude harmonic tremor began a few hours later at about 1400. The tremor was intermittent, with episodes lasting from less than a minute to 15 minutes, separated by 2- to 15-minute quiet intervals. Tremor episodes declined in number and duration after about 2 hours, and had nearly ended by 1800. On 26 September, very low-level harmonic tremor began at 0740. Minor steam emission started at 0747, lasting 9 minutes. The tremor ended by 0800. No additional tremor had been recorded as of 30 September. Outward movement of the N crater rim had virtually stopped 24 September, but measurements on the 26th indicated that very slow deformation had resumed. Although rates of outward movement have varied, the average rate for the most active portion of the N flank was slightly less than 1 cm/day during September.
In mid-September, scientists aboard a NASA P-3 aircraft studied the plume at Mt. St. Helens and material remaining in the upper atmosphere from the 18 May eruption. The aircraft left Hampton, Virginia on 17 September, flew S to Georgia, traveled W along the 32nd parallel to about Tucson, Arizona, then continued NW to Portland, Oregon. During this transcontinental flight and the return trip across the northern plains the following week, 2 wavelengths of lidar backscatter data showed a generally consistent broad layer of volcanic material at 14-22 or 23 km altitude. Peak backscatter typically occurred from two levels, 18.5-19 km and about 21.5 km. The lower was usually significantly stronger. Although the 21.5 km peak sometimes approached that at 18.5-19 km, the upper peak disappeared suddenly in certain areas.
Simultaneous data were gathered on two occasions from the P-3 aircraft and NASA's SAGE satellite, once over Sacramento, California and once over Portland. A dust sonde released from Laramie, Wyoming by James Rosen collected information on upper atmosphere particulates as the NASA aircraft made lidar measurements in the same area on its return flight.
Scientists with the NASA-funded Research on Atmospheric Volcanic Emissions (RAVE) project collected a variety of data on the plume from Mt. St. Helens during a 4-hour flight in the NASA P-3 on 22 September. Filter samples were collected and the plume was analyzed directly for SO2, H2S, OCS, CS2, O3, NO, and total S.
A University of Washington-USGS report provided additional information on the seismicity associated with the 7 August explosions and minor ash emission 8-10 August (SEAN 05:07 and 05:08). Harmonic tremor began at 1207 on the 7th, occurring as 10- to 20-second bursts at randomly spaced intervals. Low-frequency events were recorded at 1529 and 1554. At about 1622, records show a gradual increase in tremor amplitude, followed by a large seismic event at 1626:45 that marked the onset of eruptive activity. An ash-laden cloud rose to 13.5 km altitude and a small pyroclastic flow moved down the N flank. By 1730, tremor amplitude had returned to pre-1207 background levels. A series of pulsating ash ejections reaching a maximum altitude of less than 7 km coincided with a gradual increase in tremor amplitude starting at 1910. Tremor amplitude declined substantially by 2045. A second amplitude increase beginning at 2130 was followed by a large seismic event at 2232:22 as a stronger explosion sent tephra to 11 km altitude. Peak tremor levels were similar to those associated with the first 7 August explosion. At 2328, the first of eight 9-11 km-deep earthquakes occurred, all with magnitudes less than 1.7. Tremor amplitudes declined slowly. Between 8 August at 0325 and 9 August at 0129, approximately 18 events similar to large bursts of tremor and lasting 20-130 seconds were recorded. These can be correlated with discrete periods of ash emission. The last significant ash emission event was recorded on 10 August at 0813. No changes in tremor amplitude preceded these events.
Geological Summary. Prior to 1980, Mount St. Helens was a conical volcano sometimes known as the Fujisan of America. During the 1980 eruption the upper 400 m of the summit was removed by slope failure, leaving a 2 x 3.5 km breached crater now partially filled by a lava dome. There have been nine major eruptive periods beginning about 40-50,000 years ago, and it has been the most active volcano in the Cascade Range during the Holocene. Prior to 2,200 years ago, tephra, lava domes, and pyroclastic flows were erupted, forming the older edifice, but few lava flows extended beyond the base of the volcano. The modern edifice consists of basaltic as well as andesitic and dacitic products from summit and flank vents. Eruptions in the 19th century originated from the Goat Rocks area on the N flank, and were witnessed by early settlers.
Information Contacts: D. Peterson, USGS, Vancouver, WA; R.Tilling, USGS, Reston, VA; S. Malone, R. Crosson, E. Endo, University of Washington; M.P. McCormick, NASA Langley Research Center; J. Friend, Drexel University.